The present invention pertains generally to fluid flow control and, more particularly, to an actuator system for positioning a piston within a cylinder of a pneumatic circuit. The actuator system includes a uniquely configured pneumatic valving module for manipulating a flow of pressurized pneumatic fluid within the pneumatic circuit.
Actuator systems typically involve a source of compressed air that is routed through a network of pipes. The compressed air is typically provided by an air compressor that is usually driven by a motor. The compressed air is routed to a positioner that ultimately controls the flow of compressed air into and out of an actuator. The positioner provides a metered flow of compressed air into alternate ends of the actuator in response to a positioner input signal. The actuator may be a double acting actuator comprising a reciprocating piston sealed within a cylinder. The cylinder of a double-acting actuator has a working chamber on each end. The piston is slidably captured between the chambers. Both chambers of the actuator simultaneously receive and exhaust the compressed air as the piston moves back and forth within the cylinder. The piston may have a shaft extending out of one end of the cylinder with the shaft being connected to the component to be moved.
The actuator system moves or strokes the piston by forcing air into a first end of the cylinder while simultaneously withdrawing or exhausting air out of a second end of the cylinder in order to advance the piston along the length of the cylinder. Conversely, the actuator system may also force air into the second end of the cylinder while simultaneously exhausting air out of the first end of the cylinder in order to retract the piston in the opposite direction. By driving the air into alternate ends of the cylinder, the piston is moved such that the shaft can be displaced in any position for doing useful work. Actuator systems are commonly used in large scale applications such as in power plants and refineries for controlling system components such as a working valve. In such applications, it may be desirable to repeatedly position the piston to within thousandths of an inch within a very short stroking time. In addition, large scale applications may utilize large-volume actuators to react to the high forces that are typical of severe-service control valves.
When a large-volume actuator is utilized in the pneumatic circuit, the positioner, acting alone, may be unable to supply and exhaust a sufficient volume of compressed air to the actuator within a given time period. Such pneumatic circuits having large-volume actuators may be incapable of achieving a quick stroking speed of the piston. In such cases, a first and second derivative booster may be installed between the positioner and the respective first and second ends of the actuator, as illustrated in the prior art schematic of FIG. 1A. In such schematics, the positioner energizes the first and second derivative booster by providing pneumatic signals in the form of compressed air which is routed to the derivative boosters. The fist and second derivative boosters are shown enclosed within the dashed boxes of FIG. 1A. The derivative boosters allow the actuator system to achieve very short stroking times by increasing the flow rate of the positioner to the first end of the cylinder while simultaneously exhausting the second end of the cylinder through a large outlet, or vice versa. The flow rate of a device is typically characterized by the factor Cv, and may be mathematically expressed as the number of U.S. gallons of fluid per minute that will pass through a valve with a pressure drop of one psi at 60° F.
In an exemplary pneumatic circuit similar to that illustrated in FIG. 1A, the Cv of the positioner is typically greater than 0.6 with the corresponding Cv of the derivative boosters being 4.5 in the supply mode and 9.0 in the exhaust mode. The Cv of the derivative boosters in the exhaust mode is greater than the Cv in the supply mode because the exhaust capacity in a pneumatic circuit is typically the controlling factor in determining the stroking time of the piston. Continuing with the discussion of the operation of the prior art pneumatic circuit of FIG. 1A, the derivative boosters receive pneumatic signals at pneumatic pilots on either end of each derivative booster. Depending of the pneumatic signals at the pilots, the derivative boosters may be selectively opened and closed in order to regulate the flow of the compressed air into and out of the cylinder. The pneumatic pilots of the boosters are connected to the positioner through signal lines.
The derivative boosters are also connected to the air source through larger diameter feed lines. The signal lines are typically of a smaller diameter than the feed lines because they supply and exhaust compressed air into and out of the cylinder at relatively low flow rates. At higher flow rates, the positioner provides a greater flow of compressed air into the signal lines sufficient to trigger the pilots of the derivative boosters such that the derivative boosters are energized. When energized, the derivative boosters allow compressed air to flow from the larger diameter feed lines into and out of the cylinder at a higher flow rate, thereby reducing the stroking time of the piston. The prior art schematic illustrated in FIG. 1A which includes derivative boosters allows the actuator to achieve a relatively fast stroking time if the positioner has a flow rate that is high enough to energize the derivative boosters. However, where a low flow rate positioner is utilized, pneumatic circuits operating with large-volume actuators may not be able to energize the derivative booster. Consequently, they suffer the drawback of a slow stroking speed.
In many applications, it may be desirable to incorporate a lock up device into the pneumatic circuit wherein the piston may be set to fully extend or retract upon a loss of pressurization. Such a condition may result during a failure of the compressed air source. FIG. 1B is a schematic diagram similar to the prior art pneumatic circuit of FIG. 1A. FIG. 1B illustrates a pneumatic circuit that incorporates a lock up feature by including the additional components of a safety valve and first and second commutators. The first and second commutators are shown enclosed within dashed boxes in FIG. 1B. The first and second commutators are installed between the positioner and the respective first and second derivative boosters. The safety valve can also be seen in FIG. 1B as enclosed within a dashed box. The safety valve is installed between a filter regulator and the first and second commutators in parallel to the positioner.
Advantageously, in FIG. 1B, the safety valve and first and second commutators provide a fail safe feature wherein the actuator may be set to lock up into a fail open or fail close position upon a loss of pressurization within the pneumatic circuit. In the fail close condition, the piston is displaced toward one end of the cylinder such that the shaft of the piston is extended in order to close a working valve that may be connected to the shaft. Alternatively, in the fail open position, the piston is displaced toward the opposite end of the cylinder such that the shaft of the piston may be retracted in order to open the working valve. Although it is advantageous to incorporate a lock up feature within a pneumatic circuit, the additional components of the safety valve and the first and second commutators, as shown in FIG. 1B, unfortunately reduce the piston stroking speed.
In some applications, the flow rate of the positioner may be quite small such that the derivative boosters may not be energizable by the relatively small pneumatic signals sent by the positioner. For example, the pneumatic circuit of FIG. 1C incorporates a positioner with a Cv of less than 0.6. With such a low flow rate positioner, a pair of volume boosters may be added into the pneumatic circuit to amplify the positioner signal. In FIG. 1C, first and second volume boosters are located between the first and second commutators, respectively. The volume boosters amplify the relatively low flow rate of the positioner. In comparison to the relatively large Cv of the derivative boosters, the Cv of the volume boosters is typically only about 1.0 in the supply mode and about 1.0 in the exhaust mode. However, operating in conjunction with the volume boosters, the derivative boosters may be energized such that the flow rate into and out of the cylinder may be greatly increased. Such increased flow rate provides the actuator system with a markedly increased stroking speed of the piston. In addition, the pneumatic circuit of FIG. 1C includes the additional benefit of including the safety valve and the first and second commutators for the lock up feature.
However, the benefits that are provided by the additional first and second derivative boosters, the safety valve and the first and second commutators in FIG. 1C are accompanied by a performance penalty. In pneumatic circuits having a large number of active components, dynamic instability occurs wherein the piston is difficult to precisely and rapidly position. For example, in the pneumatic circuit of FIG. 1C, the active components include the positioner, the safety valve, the pair of derivative boosters and the pair of volume booster all connected to first and second ends of the cylinder. As a result of the maze of piping and fittings interconnecting the many active components, the total requirement of compressed air out of the positioner that is needed in order to effect a given piston movement is increased compared to pneumatic circuits having a lesser number of active components. Furthermore, due to the inherently compressible nature of air, the piston may not start to move toward the desired position until the pair of derivative boosters and the pair of volume boosters have sufficiently pressurized.
Thus, there may be an undesirable lag between the time that the positioner receives the piston position signal and the time that the piston arrives at the desired position. Also, due to the amplification chain in successively energizing the derivative and volume boosters, the piston may overshoot the final position. Overshooting occurs when the piston, moving at a relatively high rate of speed, fails to slow down as it nears the final position such that it moves past the desired position and must then reverse directions. The overshooting of the piston therefore increases the overall lag time of the actuator.
As can be seen, there exists a need in the art for an actuator system having a large-volume actuator wherein the piston has a relatively short stroking time. Also, there exists a need for an actuator system having a large-volume actuator wherein overshooting of the piston may be minimized or eliminated. In addition, there exists a need for an actuator system wherein the total requirement of compressed air out of the positioner is minimized. Furthermore, there exists a need in the art for an actuator system wherein the interactive effects of the boosters on the piston may be eliminated. Finally, there exists a need in the art for a pneumatic control system that may be retrofitted into existing pneumatic circuits.